I have seem some comments that India was bigger economy than France, using Maddison's data, which leads to the conclusion that India was a bigger colonial asset than France. This points out to problems in using Maddison's datasets. One has to take into account also the level of economic development of the country under consideration and how those estimates were constructed. A good idea is to check the size of India's machine tool stock compared to France's, I suspect there was a significant difference, or per capita income.
For instance, for 1939 we have the following GDP data (Europe excludes the Soviet Union) from Madison (adjusted with Figure 1 from http://gpih.ucdavis.edu/files/Fukao_Ma_Yuan.pdf, for better estimates for China and Japan), I also adjusted Europe's using Germany's per capita GDP for the 79.3 million inhabitants of the Reich instead of 68.6 million and deleting Austria and reducing Czechoslovakia pop. by 1/4 from the dataset (since Maddison's estimate is based on national account data and in 1939 there wasn't any for Austria or 1/4 of Czechoslovakia or Germany in the old borders), though overall change for Europe is 1%:
Europe -------------- 1,567,815
USA ------------------ 862,995
USSR ----------------- 430,314
China ---------------- 288,653
India ----------------- 256,924
Japan ---------------- 166,506
From this data it superficially appears that China and India were big economies with major impact on the world economy and great sources of resources to fight the war. However, one needs to look a bit into the per capita income estimates to get a more sensible picture:
Europe -------------- 4,045
USA ------------------ 6,561
USSR ----------------- 2,237
China ---------------- 619
India ----------------- 674
Japan ---------------- 2,301
The only large industrialized regions of the world were Europe and the US, these two regions, while accounting for nearly 60% of the world's GDP, had way more than 60% of the world's warmaking potential, more likely around the region of 80% (with USSR and Japan together accounting for most of the other 20%). And Europe here includes Spain, Yugoslavia, Greece, Portugal, and other non-industrialized regions, removing them from the picture (they represent ca. 5% of it's GDP in Madison's estimate), changes nothing besides increasing Europe's average GDP to ca. 4,500 dollars, or 70% of the US's level.
One should also think about how those estimates were constructed. China and India already had PPP per capita levels less than 1/10 of the highest per capita income in 1939 (Switzerland), how those were calculated? Well, Germany's per capita income was 5,500 dollars, average labor wages were 1,850 RM (in 1937), a kilo of bread with 4,000 calories cost 0.31 RM. Minimum subsistence wages were usually of 8,000 calories, or 2 kilos of bread a way, or 700 kilos of bread a year, that's 217 RM, or about 1/8 of Germany's average wages. Look again at the per capita income estimates:
Germany ------------ 5,500
China ---------------- 619
India ----------------- 674
China and India were minimum subsistence agricultural economies: 80-90% of the population consisted of subsistence farmers who produce mostly for self consumption hence most output wasn't sold at the market, it wasn't a monetary society nor one which people needed to develop skills such as reading and mathematics (hence, nearly all the population was illiterate), but a society were each farmer worked 60 hours a week to produce a minimum amount of food required for survival. Life expectancy at birth was ca. 25 years, compared to 65 years for Germany and 45 years for the Soviet Union. Their total GDP is estimated by simply multiplying the cost in Germany or the UK, or France of a minimum subsistence basket and multiplying that price by the population of India and China. They couldn't produce any war related munitions nor supply their armies with basic necessities: The output of steel in China not under Japanese occupation was estimated at 10,000 metric tons.
British colonization implemented a small modern sector in India which is the reason I suspect it's per capita income is slightly larger but notice that this modern sector could only affect a tiny proportion of the total population: in 1913, Indian manufacturing production was 13 times smaller than Britain's, while it's population was 7 times larger, per capita manufacturing output was 1/100 of British level meaning that only a fraction of perhaps around 1-2% of the population of India was living in a modern industrialized society versus the 98-99% of subsistence farmers. Hence, one should be cautious when one thinks about estimating that India had larger warmaking potential than France because it had a larger output measured in minimum subsistence baskets (just because it's huge population was alive). In fact, both India and China were not significant in terms of warmaking potential.
And Japan and the USSR were half industrialized societies. Thus their warmaking potential was relatively lower than GDP if compared to Europe or the US. The USSR had a similar level of GDP to Germany in 1939:
USSR: 370 (excluding territories the USSR gained after the war from Eastern Europe which were included in the table above)
Germany: 435 (taking into account Germany's annexations)
But measured in terms of output of basic industrial goods it was always lower than Germany's. The stock of lathes in 1938 was 1/6 of Germany's as well as output of machine tools during the war (though machine tool stocks tends to overestimate Germany's industrial resources since the country's economy was specialized in metal working industries before the war). In 1937, the per capita outputs of 7 industrial commodities: coal,pig iron, steel, electricity, soap (indicator of development of chemical industry), paper, cement was on average 25% of Germany's level (while per capita income was 40%), given it's population was slightly more than twice Germany's (168 to 79 million in 1939), so total level of industrial production of these commodities was ca. 55% of Germany's. After Barbarossa, the Soviet population dropped by 40% and many critical industrial areas were lost, so it's level of industrial production dropped further, in terms of these industrial commodities, to ca. 30% of Germany's level, less if we include areas under German occupation.
Japan's case is similar, it's estimated PPP GDP in 1939 was ca. 20% of the US's, but it's steel output was 10% and it's GDP on market exchange rates was 8.7%. Per capita levels of Japan's industrial production in these 7 industrial commodities were 20% of the US's level in 1937 while estimated GDP per capita was 35%. In terms of total level of industrial output of these commodities was ca. 11% of US's level. Germany's average output of these 7 industrial commodities was 83% of the US's in 1937, while estimated PPP per capita income was 84% in 1939. For Europe and the US overall (disconsidering the tens of millions of people in non-industrialized countries in Europe), GDP reflected industrial/warmaking potential better in this case.
So, overall, in terms of industrial warmaking potential it appears we have the following relationship, with China and India around 10% of it's PPP GDP to reflect the small size of the monetary (hence, taxable, thus, relevant for the British officers or Japanese occupators) sector of their economies, Europe we discount the non-industrialized countries (Romania, Portugal, Spain, Greece and Yugoslavia), the USSR and Japan are adjusted to 60% of their GDP level to match per capita supply of industrial commodities and the USSR is corrected also for territories which they gained after the war (Maddison uses post-war borders):
Europe -------------- 1,450,000
USA ------------------ 862,995
USSR ----------------- 223,000
Japan ---------------- 100,000
China ---------------- 29,000
India ----------------- 27,000
USA+Europe portion of the sample of regions increases from 65% to 85%, a more accurate representation. I don't think having enormous amount of colonies helped France and UK in their war versus Germany (with the important exception of UK's industrialized colonies: Canada and Australia, though these two countries had 30% of the economic size of the UK at the time, Canada's economic size was estimated 1/6 of the UK's and it's output of industrial products such as steel, coal and aircraft was even lower in comparison: Canada produced 1.6 million tons of steel a year on average (1/7 of UK's), 15 million tons of coal and lignite (7% of UK's) and 3,000 aircraft (1/8 of UK's output), it's warmaking potential was very small compared to the UK, Australia's industrial production was even more nimble.
War-making potential of the coalitions were:
Axis
Germany - 430,000
Italy ------ 110,000 (80% of it's estimated PPP GDP, since Italy was more industrialized (yes) than Japan or the USSR, possessing half of the PPP US's per capita income in the 30's instead of 1/3 for Japan or the USSR)
Japan ----- 100,000
Allies
US -------- 860,000 (or perhaps 950,000 adjusting for the very high rate of unemployment due to the depression: increasing hours worked in the country by 15%, taking into account the decreasing returns of increasing hours worked over a given capital stock)
UK -------- 300,000
USSR ----- 223,000
Italy, though, never mobilized for total war: military expenditures were around 15% of it's GDP before it surrendered, Germany and Japan managed to increase their's to 70% in 1944. The USSR managed to mobilize very effectively it's relatively small warmaking potential, unlike the US and the UK, who had vast industrial resources and spend most of them on bombers and naval vessels, the USSR focused their smaller resources (specially after Barbarossa) on producing basic military equipment: rifles, machine guns, tanks, cannons. Stuff that in proportion to the military expenditures of Germany, UK and the US were very small (in Germany in 1943, expenditures on tanks, small arms and artillery pieces was 3.6 billion RM, out of 112 billion RM military outlays: providing basic equipment for the army was almost trivial for Germany, a great effort for the USSR). Though the type of war the USSR fought favored it's superior manpower, in the US/Japan case, manpower was almost irrelevant (as long as you could man the ships and the few divisions needed to capture islands).
The only territories outside these 6 countries with significant warmaking potential were in continental Europe (Canada was the largest industrial producer outside of Europe, US, USSR and Japan, it's industrial capacity was nearly an order of magnitude smaller than the UK's), which Germany dominated, but output in these regions collapsed, overall Germany's warmaking capacity addition given by occupied territories was around 25% (the proportion of Germany's military expenditures paid by them).
I believe that today warmaking potential is much more evently distributed around the world. As industrialization has spread to many countries, Brazil, for example, today is a major power and industrial producer, in WW2 it was irrelevant: in 2013 Brazil produced 3.7 million motor vehicles and 35 million tons of steel, in 1939, Brazil produced 0.11 million tons of steel and 0 motor vehicles. In that sense, even though the US's proportion of world PPP GDP has stayed roughly constant at 20% in both 1939 and 2013, US's proportion of warmaking potential has declined, from ca. 30% in 1939 to 17% (current US's proportion in manufacturing output), in 2012.
. These expenditures averaged 75% of Germany's national income during the 68 months of the war (Germany's national income in these 68 months was ca. 820 billion RM). I assumed that 90% of these expenditures were military, although only 414 billion RM were direct Werhmacht expenditures I also assumed that 140 billion RM out of 200 billion RM of expenditures in civilian deparments were military expenditures (i.e. the Atlantic Wall).